The present invention relates to a microwave oven for heating foodstuffs with a browning device according to the preamble to claim 1.
Microwave ovens for heating foodstuffs which are provided with a browning device are already available. The browning device serves to give the foodstuffs a browned surface while the essential heating is achieved by microwaves that are fed to the foodstuffs from a microwave unit. As a rule, the browning device consists of an omnidirectional radiation means which generates infrared (IR) radiation combined with a metal reflector for directing IR radiation towards the foodstuffs.
The grill element is conventionally arranged in a grill bulge outside the oven cavity to prevent the microwave pattern in the cavity from being interfered with. To permit the IR radiation to leave the grill bulge, an opening in the wall of the cavity must be arranged, through which microwave radiation can unfortunately leak from the cavity.
Swedish patent application 9700280-2 discloses a device and a method for preventing microwave radiation from leaking through the grill bulge, by the grill bulge and its connection opening being formed as a waveguide with such dimensions that its properties in respect of microwave propagation are such as to allow the space to be essentially free of microwaves.
Browning devices with reflectors are usually provided with a protection means protecting against fat splashing from the foodstuffs since fat deposited on the reflector essentially deteriorates its reflectance of IR radiation and a larger amount of IR radiation will be absorbed by the surface. The increased absorption results in an increased temperature of the reflector, which in turn leads to a further deterioration of the reflectance. The protection means in front of the browning device is usually designed as a grating placed between the reflector and the oven cavity. The grating can be designed to absorb IR radiation from the grill element such that it obtains a high temperature. This results in the formation of a hot zone round the grating where the fat is burnt, thus avoiding that the fat deposits on the reflector and consequently deteriorates its reflectance.
A drawback of the grating is that an increased power of the browning device is required to compensate for the power drop in the protective grating. This increased power consumption should be added to the high consumption of power of the browning device as it is. Moreover, the ovens that are presently available frequently require two browning devices to obtain sufficient IR radiation efficiency.
Increased power of the browning device means that the power consumption of the oven increases and that the power supply need be reinforced and also that more power must be cooled away, which places greater demands on the cooling system. This results in the ovens becoming more expensive.
A further problem is the leaking of microwave radiation from the oven cavity to the grill bulge, which has not been completely eliminated by the prior art solutions.
One more problem is that the connection opening between the grill bulge and the oven cavity interferes with the field pattern of the oven cavity.
There is thus a need for providing a microwave oven with a grill element having a lower consumption of power, where the browning device is designed in such manner that its negative effect on the microwaves in the oven cavity is reduced and the heat loss is reduced.
An object of the present invention is to provide a microwave oven with a grill element, whose negative effect on the function of the microwave oven is reduced.
A further object of the present invention is to provide a microwave oven with a reduced consumption of power, a reduced need for cooling, the leakage of microwave radiation from the cavity being reduced as well.
These objects are achieved by a device of the type mentioned by way of introduction, which has the features defined in claim 1. Further preferred features of the inventive device are recited in the dependent claims.
A basic idea of the invention is to use a heat-resisting material for the reflector.
By using a browning device with a reflector which has at least a surface layer of a non-metallic, heat-resisting, reflective material, the distance between the radiation means and the reflector can be made considerably smaller than in the case where metallic reflectors are used, and thus also the browning device can be made smaller. According to the invention use is preferably made of a material which retains its reflective properties at a temperature of typically at least 500xc2x0 C., and preferably at least 800xc2x0 C.
Moreover the reflector can be designed so as to achieve a generally improved directive efficiency. It can thus be avoided that direct radiation from the radiation means falls on the door of the microwave oven. Using a metal reflector, the necessarily great distance between the reflector and the radiation means would result in the browning device being huge to make the geometry such that the direct radiation from the radiation means does not fall on the door of the oven.
According to one aspect of the invention, a browning device is provided, which essentially illuminates the loading zone by the radiation means being placed in a reflector which is designed to screen off radiation from the radiation means such that it does not fall on the door of the oven. The reflector has a concave surface with an opening. Radiation from the radiation means will be spread at an angle after having passed the opening. The angle depends on the distance between the opening and the radiation means.
The reflector is designed with two preferably essentially parallel sides and a suitably rounded base. This design of the reflector is favourable from the viewpoint of manufacture and results in relatively good reflective properties. By the reflector being made narrow and deep compared with today""s reflectors, the possibilities of screening off direct radiation from the radiation means will be improved.
According to one aspect of the present invention, the browning device is arranged at the rear edge of the top of the oven cavity. This arrangement makes the browning device well protected from being mechanically affected in spite of a favourable IR radiation.
According to a further aspect of the present invention, an arrangement of the browning device at the rear edge of the top of the oven cavity, furthest away from the door, is combined with an arrangement of the foodstuffs on a rotary plate. Preferably, the reflector is designed such that the maximum radiation intensity of the rotary plate is to be found outside the centre and preferably midway between the centre of the rotary plate and its rear edge. As the plate rotates, the average radiation intensity will essentially be uniform over the entire surface of the rotary plate.
In case the radiation source is extended, only part of the radiation from the radiation source will be screened off in certain directions. The intensity of the direct radiation falling on the rotary plate depends on the one hand on the distance to the radiation source and, on the other hand, on the amount of radiation that has been screened off. Preferably, the design and position of the browning device is arranged such that the surface on which radiation from the entire radiation means falls is to be found in the rear part of the oven cavity. The surface on which direct radiation from the entire radiation means falls is also defined by the fact that there is a straight line that does not pass any obstacle from each point of the surface to each point of the radiation means.
Alternatively, the browning device can be arranged at the front edge of the top of the oven cavity closest to the door, in which case the surface on which direct radiation from the entire radiation means falls is positioned between the centre and the front edge of the rotary plate.
According to one more aspect of the invention, the browning device is arranged in a grill bulge with a connection opening to the oven cavity. By placing the reflector adjacent to the radiation means, its dimensions can be small. With small dimensions of the reflector, the connection opening can be narrow, which results in a reduced leakage of microwave radiation from the oven cavity to the grill bulge and further out of the oven.
The grill bulge is advantageously arranged above the top of the oven cavity at the rear edge thereof furthest away from the door.
If the entire reflector is made of a non-metallic material, the reflector can be placed in the oven cavity without the microwaves in the oven cavity being affected to a considerable extent.
According to a further aspect of the present invention, use is made of a material at least in a reflective surface layer such that it reflects at least 50% and preferably 70% of the incident radiation.
High reflectivity is achieved according to one aspect of the invention by at least a surface layer of the reflector being made of compacted fibres or grains, of a dielectric material having a high refractive index for IR radiation. The refractive index of the dielectric material is at least 1.5 and preferably above 2 for IR radiation. The essential thing is that the reflector comprises a large number of surfaces in which refraction or reflection occurs. A similar result can be achieved by having a plurality of small particles having a high refractive index spread in a material having a lower refractive index, or small particles having a low refractive index in a material having a higher refractive index. Spreading in the small particles will then be achieved.
According to one aspect of the invention, at least a surface layer of the reflector is essentially made of calcium oxide, calcium sulphate, silica, barium sulphate, zirconium oxide or titanium oxide.
According to one aspect of the invention, the surface layer of the reflector is essentially made of a mixture of a selection of calcium oxide, calcium sulphate, silica, barium sulphate, zirconium oxide and titanium oxide.
In a mixture of a selection of calcium oxide, calcium sulphate, silica, barium sulphate and titanium oxide, it is possible that also some other substance is included to improve the mechanical properties of the surface layer.
By using, according to the invention, a radiation means having a temperature of between 1100xc2x0 C. and 1700xc2x0 C. and preferably between 1300xc2x0 C. and 1500xc2x0 C., an increased radiation yield will be obtained compared with the case in which a lower temperature is used. By increasing the temperature from the normally employed temperature 800xc2x0 C., it is thus possible to reduce the radiating surface of the radiation means with the radiated power retained. Consequently, the dimensions of the radiation means can be reduced, and moreover only one browning device is necessary to achieve sufficient power. Thus, an inventive device will have great advantages although the somewhat shorter wavelength from a radiation means having a high temperature produces a somewhat poorer grilling result.
A high temperature of the filament is produced according to one aspect of the invention by using a halogen bulb, a quartz tube or the like.
According to a further aspect of the invention, use is made of a material having a low thermal conductivity to reduce the thermal conduction to the casing. This reduces the cooling requirement and also gives the advantage that the temperature of the surface of the reflector can be kept high, which results in fat splashing onto the reflector being burnt off. This results in a self-cleaning function and no protective grating is required, which entails reduced power loss. The reflector surface should have a temperature of at least 500xc2x0 C. for the self-cleaning effect to be optimal.
An unexpected and surprising advantage of having a reflector surface with poor thermal conductivity is that the reflector obtains a high temperature, which makes it function as an IR radiator, which results in an increased radiation yield since the radiation absorbed in the reflector partially radiates back. The somewhat lower temperature of the reflector compared with the temperature of the radiation means results in the wavelength of the radiation from the reflector being in a range which is favourable in terms of grilling.
The above aspects can, of course, be combined in the same embodiment.
In the following, detailed exemplifying embodiments of the invention will be described with reference to the Figures.